The present invention relates to a push button switch for use with various input devices of a computer, word processor, etc.
A related conventional push button switch is disclosed in Japanese Utility Model Laid-open Publication No. 61-23238, for example, as shown in FIGS. 6 and 7.
Referring to FIG. 6 which is a vertical sectional view of this prior art push button switch, the push button switch is constructed primarily of a casing unit 1 composed of an upper casing 2 and a lower casing 3, a stem 5 slidably supported by the upper casing 2 and partially projecting from an upper opening 4 of the upper casing 2, a compression spring 6 interposed between the lower casing 3 and the stem 5, a switch device 7 adapted to be switched on and off in association with sliding motion of the stem 5, and a clicking leaf spring 8 having a free end abutting against the stem 5 during the sliding motion of the stem 5 for giving a click feeling to an operator.
As shown in FIG. 7, the clicking leaf spring 8 is comprised of a base portion 9 which is in contact with an inner side surface of the upper casing 2, and a spring portion 10, formed by bending the clicking leaf spring at an upper end of the base portion 9 such that spring portion 10 extends obliquely downward to contact stem 5. At the free end of spring portion 10 is formed a pair of stepped portions 11 for giving a click feeling to the operator. The base portion 9 is formed at its opposite sides with a pair of elastic projections 12 adapted to elastically abut against inner surfaces of grooves (not shown) formed in the upper casing 2, so that the base portion 9 is securely held against the inner side surface of the casing 2. The elastic projections 12 are formed by deforming a pair of tabs split from a central portion of the base portion 9 forming a pair of vertical slits between the projections 12 and the base portion 9. As shown in FIG. 6, a bent portion 14 formed between the base portion 9 and the spring portion 10 abuts against an inner surface of the upper casing 2, and a lower end of the base portion 9 abuts against an inner surface of the lower casing 3. Thus, the base portion 9 is bracketed between the upper and lower casings 2 and 3 and thus prevented from moving along its length.
In operation, when the stem 5 is depressed from its rest position shown in fig. 6, a plate portion 16 of the switch device 7 is elastically deformed in a counterclockwise direction as viewed in fig. 6 (in the direction as depicted by arrow A), while the spring portion 10 of the clicking leaf spring 8 is also elastically deformed in a clockwise direction as viewed in fig. 6 (in the direction as depicted by arrow B). When the stem 5 is further depressed to reach a position near a lowermost position, contacts (not shown) of the switch device 7 are closed by the deformation of the plate portion 16. At the same time, the lower end of the stem 5 is brought into abutment against the stepped portions 11 of the spring portion 10, and rides over the same, causing a change in depression force and thereby giving a click feeling to the operator.
Such a click feeling depends on an elastic modulus of the clicking leaf spring 8 itself and a span S.sub.1 between the bent portion 14 and the stepped portions 11. Therefore, when the push button switch is made more compact, the span S.sub.1 is reduced to naturally cause an increase in spring force of the spring portion 10. The increased spring force can cause both the breakage of the portion of stem 5 in contact with the stepped portion 11, and increased friction wear between the stem 5 and slide path defined by casing 1.
To cope with this problem, it has been considered to reduce the thickness of the clicking leaf spring 8, so as to reduce the spring force and therefore reduce the elastic modulus. However, although a suitable spring force can be obtained by reducing the thickness of the leaf spring 8, the mechanical strength of the leaf spring 8 is reduced and generates a possibility of breakage due to repeated stress.